Droplet ejection devices are used for a variety of purposes, most commonly for printing images on various media. Droplet ejection devices are often referred to as ink jets or ink jet printers. Drop-on-demand droplet ejection devices are used in many applications because of their flexibility and economy. Drop-on-demand devices eject one or more droplets in response to a specific signal, usually an electrical waveform that may include a single pulse or multiple pulses. Different portions of a multi-pulse waveform can be selectively activated to produce the droplets.
Droplet ejection devices typically include a fluid path from a fluid supply to a nozzle path. The nozzle path terminates in a nozzle opening from which droplets are ejected. Each ink jet has a natural frequency which is related to the inverse of the resonance period of a sound wave propagating through the length of the ejector (or jet). The jet natural frequency can affect many aspects of jet performance. For example, the jet natural frequency typically affects the frequency response of the printhead. Typically, the jet velocity remains near a target velocity for a range of frequencies from substantially less than the natural frequency up to about 25% of the natural frequency of the jet. As the frequency increases beyond this range, the jet velocity begins to vary by increasing amounts. This variation is caused, in part, by residual pressures and flows from the previous drive pulse(s). These pressures and flows interact with the current drive pulse and can cause either constructive or destructive interference, which leads to the droplet firing either faster or slower than it would otherwise fire.
One prior ink jetting approach uses a pulse string followed by a cancelling pulse. The cancelling pulse is a shortened pulse that is timed so that the resulting pressure pulses arrive at the nozzle out of phase with the residual pressure from previous pulses. Given that jets will have a dominant resonant frequency, the cancellation features are timed in units of resonance period Tc.
Droplet ejection devices need to generate drops sustainably, obtain a required drop volume, deliver material accurately, and achieve a desired delivery rate. Drop placement errors with respect to a target degrade image quality on the target. FIG. 1 illustrates different types of drop placement errors. A drop 121 is fired through a nozzle plate 110 towards a target 130. Vertical line 171 represents an ideal straight drop trajectory. However, a nozzle error 141 results from a misalignment of the nozzle with respect to the target. Vertical line 180 represents a straight drop trajectory from the nozzle to the target with this line being orthogonal to the nozzle plate 110. An angle theta formed between the vertical line 180 and the actual trajectory 190 of the drop represents the jet trajectory error 151. A total drop placement error 161 equals the combination of nozzle placement error and jet trajectory error.